1. Field of the Invention
This invention relates to a flat image display device provided with a pair of substrates opposed to each other.
2. Description of the Related Art
Various flat image display devices have been developed as a next generation of image display devices in which a large number of electron emitting elements are arranged side by side and opposed to a phosphor screen. While there are various types of electron emitting elements for use as electron emission sources, all of them basically utilize field emission. Display devices that use these electron emitting elements are generally called field emission displays (hereinafter, referred to as FED's). Among the FED's, a display device that uses surface-conduction electron emitting elements is also called a surface-conduction electron emission display (hereinafter, referred to as an SED). In this specification, however, the term “FED” is used as a generic name for devices including the SED.
In general, an FED comprises a front substrate and a rear substrate that are opposed to each other with a given gap between them. These substrates have their respective peripheral portions joined together by a sidewall in the shape of a rectangular frame, thereby constituting a vacuum envelope. The interior of the vacuum envelope is kept at a high vacuum such that the degree of vacuum is about 10−4 Pa or below. In order to support an atmospheric load that acts on the front substrate and the rear substrate, a plurality of support members are located between these substrates.
A phosphor screen that includes red, blue, and green phosphor layers is formed on the inner surface of the front substrate, and a number of electron emitting elements that emit electrons for exciting the phosphor to luminescence are provided on the inner surface of the rear substrate. Further, a number of scan lines and signal lines are formed in a matrix and connected to the electron emitting elements. An anode voltage is applied to the phosphor screen, and electron beams emitted from the electron emitting elements are accelerated by the anode voltage and collide with the phosphor screen, whereupon the phosphor glows and displays an image.
In the FED of this type, the gap between the front and rear substrates can be set to several millimeters or less. When compared with a cathode-ray tube (CRT) that is used as a display of an existing TV or computer, therefore, the FED can achieve lighter weight and smaller thickness.
In order to obtain practical display characteristics for the FED constructed in this manner, it is necessary to use a phosphor that resembles that of a conventional cathode-ray tube and to use a phosphor screen that is obtained by forming a thin aluminum film called a metal back on the phosphor. In this case, the anode voltage to be applied to the phosphor screen is set to at least several kV, and preferably, to 10 kV or more.
In view of the resolution, the properties of the support members, etc., the gap between the front substrate and the rear substrate cannot be made very wide and is set to about 1 to 2 mm. In the FED, therefore, a strong electric field is inevitably formed in the narrow gap between the front substrate and the rear substrate, so that electric discharge between the substrates raises a problem.
If no countermeasures are taken to restrain electric discharge damage, electric discharge inevitably causes breakage or degradation of the electron emitting elements and their connected thin-film electrodes, phosphor screen, driver IC, and drive circuit. These phenomena will be referred to collectively as electric discharge damage. In a situation that involves such damage, electric discharge must be absolutely prevented from being generated for a long period of time in order to put the FED into practical use. However, it is very difficult to realize this.
Accordingly, it is essential to take a countermeasure to reduce the discharge current so that electric discharge, if any, can be restricted to a level such that no or negligible electric discharge damage occurs. A technique to attain this is described in Jpn. Pat. Appln. KOKAI Publication No. 2000-311642. According to this technique, a metal back on a phosphor screen is notched to form a zigzag or other pattern, whereby the effective impedance of the phosphor screen is enhanced. Described in Jpn. Pat. Appln. KOKAI Publication No. 10-326583, moreover, is a technique in which a metal back is divided and connected to a common electrode through a resistance member so that high voltage can be applied. Described in Jpn. Pat. Appln. KOKAI Publication No. 2000-251797, furthermore, is a technique in which divided parts of a metal back are coated with an electrically conductive material to restrain discharge at the divided parts. Described in Jpn. Pat. Appln. KOKAI Publication No. 2003-242911 is a technique in which a metal back is divided or patterned, and moreover, a resistive material is used for the metal back.
However, a continued examination has revealed that the discharge current can be reduced only to about 3 A by such technique, among other prior art techniques, in which the metal back is divided in the longitudinal direction with a high discharge current limiting effect.
Thus, breakage of the phosphor screen and the driver IC can be prevented. Electron sources can be prevented substantially securely from being damaged. If any electric discharge that involves the electron emitting elements takes place, though rarely, however, point defects may occur in some cases. Further, a countermeasure to restrain disconnection of the thin-film electrodes that are connected to the electron emitting elements causes an increase of processes in number and cost increase. On the other hand, the driver IC must be specially designed to cope with a current of about 3 A, so that cost increase is caused. Accordingly, there has been an increasing demand for a technique capable of reducing the discharge current.